The present invention relates generally to convolutional codes for use in communication systems, and more particularly to punctured convolutional codes optimized for use in conjunction with digital audio broadcasting and other types of communication system applications which utilize diversity in frequency, time, space, polarization or other system parameters.
FIG. 1 shows a portion of a frequency spectrum in an exemplary In Band On Channel (IBOC) system for implementing digital audio broadcasting (DAB) in existing analog frequency modulation (FM) radio bands. In this IBOC system, an analog FM carrier signal 10 serves as a xe2x80x9chostxe2x80x9d for transmission of digital audio information of CD-like quality. The same digital audio information is transmitted on both a lower sideband 12 and an upper sideband 14 of the analog host 10, using a multicarrier OFDM technique. This ensures that all of the digital information can be recovered when one of the sidebands is corrupted, or even completely lost, due to effects such as fading or interference in the crowded analog FM band. The digital audio subcarriers transmitted in region B of the lower and upper sidebands 12, 14 are generally less susceptible to interference from adjacent FM channels or the analog host 10 than the carriers in regions A or C. The subcarriers in region A of sidebands 12, 14 are more susceptible to adjacent channel interference, while those in region C are more susceptible to interference from the analog host 10. The transmission in region C may make use of precancellation techniques which allow the interference with the analog host 10 to be canceled. Additional details regarding this exemplary IBOC system can be found in B. W. Kroeger and A. J. Vigil, xe2x80x9cImproved IBOC DAB Technology for AM and FM Broadcasting,xe2x80x9d SBE Engineering Conference, pp. 1-10, 1996.
It has been proposed that complementary convolutional codes be utilized for channel coding in DAB systems such as the IBOC system described in conjunction with FIG. 1. For example, a pair of complementary codes can be used individually on both sides of the analog host 10 in the system of FIG. 1. A pair of complementary codes can be generating by xe2x80x9cpuncturingxe2x80x9d a low-rate xe2x80x9cmotherxe2x80x9d code to twice its original rate. Puncturing a mother code is a well-known technique for obtaining high-rate convolutional codes which exhibit good performance and which can be decoded using the same basic Viterbi algorithm that is used for the mother code. See, for example, G. C. Clark, Jr. and J. B. Cain, xe2x80x9cError Correcting Codes for Digital Communications,xe2x80x9d Plenum Press, 1981, S. Lin and D. J. Costello Jr., xe2x80x9cError Control Coding: Fundamentals and Applications,xe2x80x9d Prentice-Hall, 1983 and Y. Yasuda, K. Kashiki and Y. Hirata, xe2x80x9cHigh-rate punctured convolutional codes for soft decision Viterbi decoding,xe2x80x9d IEEE Transactions on Communications, Vol. 32, March 1984.
Puncturing generally involves removing bits from the low-rate mother code such that the remaining code bits form one of the complementary codes, while the punctured bits form the other complementary code of the code pair. The resulting pair of codes, which are referred to as complementary punctured-pair convolutional (CPPC) codes, are complementary in the sense that they are both of a rate which is twice that of the mother code obtained by combining the two codes; Increased puncturing leads to higher punctured code rates. It can be shown that punctured codes of a certain rate generally provide performance which is almost as good as that of optimal codes at the same rate. Unfortunately, the conventional CPPC codes which have been proposed for use in the IBOC system described in the above-cited B. W. Kroeger and A. J. Vigil reference generally do not provide optimal or near-optimal performance, and in some cases are even catastrophic. This may be due in part to a perceived requirement that the code pairs be so-called xe2x80x9cequivalentxe2x80x9d codes, as defined in S. Kallel, xe2x80x9cComplementary Punctured Convolutional Codes and Their Applications,xe2x80x9d IEEE Transactions on Communications, Vol. 43, No. 6, pp. 2005-2009, June 1995, which is incorporated by reference herein. However, this perceived requirement has had the effect of unduly restricting the scope of search for CPPC codes. A need therefore exists for improved punctured convolutional codes which can provide better performance than conventional codes in the above-described IBOC digital audio broadcasting system and other applications.
The invention provides optimal punctured convolutional codes for use in digital audio broadcasting as well as other types of communication systems. Optimal punctured convolutional codes are provided for equal error protection (EEP) applications, and both rate-compatible and rate-incompatible codes for are provided for unequal error protection (UEP) applications. In an exemplary embodiment, an optimal code is selected as a code which has the best free Hamming distance and the minimum information error weight from among a set of potential non-catastrophic codes for a given set of operating parameters. Unlike conventional punctured code sets used for digital audio broadcasting applications, a set of potential non-catastrophic codes in accordance with the invention can include codes which are not equivalent in terms of their distance or performance properties. The selected optimal code thus provides performance advantages relative to a code selected from a set restricted to only equivalent codes. Although particularly well suited for use with complementary code pairs, the techniques of the invention can be readily extended for use in selecting an optimal group of n complementary codes from a set of such groups.
The invention may be implemented in an exemplary system in which digital audio information is transmitted on subcarriers in both an upper and a lower sideband of an analog carrier. In such a system, an optimal complementary code pair is selected from a set of code pairs defined in the manner described above. The complementary codes in the selected code pair may each be, for example, a rate-4/5 half-bandwidth convolutional code which is generated by puncturing a rate-2/5 full-bandwidth convolutional code. The full-bandwidth code may itself be generated by puncturing a rate-1/3 mother code. The invention also provides an optimal bit assignment strategy for use in such a system. In accordance with this strategy, bits from a designated code generator may be assigned to the upper and lower sideband subcarriers which are located furthest from the analog carrier. These and other techniques of the invention can be readily extended to a wide variety of different types of communication systems. For example, the invention can be implemented in communication system applications which utilize diversity in frequency, time, space, polarization or any other system parameter.